High luminance polarizing plate, and liquid crystal panel and image display using same

ABSTRACT

A high-brightness polarizing plate of the present invention comprises a polarizing plate that comprises a polarizer and a protective film prepared on one or both sides of the polarizer, a brightness enhancement film and an adhesive layer through which the polarizing plate and the brightness enhancement film are laminated with the protective film interposed between them, wherein the protective film has an in-plane retardation Re of 0 to 10 nm and a thickness-direction retardation Rth of −30 to 10 nm, wherein Re=(nx−ny)d and Rth={(nx+ny)/(2−nz)}d. The high-brightness polarizing plate has a small color shift, and can be applied to various image displays such as liquid crystal displays.

TECHNICAL FIELD

The invention relates to a high-brightness polarizing plate comprising alaminate of a polarizing plate and a brightness enhancement film. Thehigh-brightness polarizing plate of the invention may be used alone orin combination with any other optical film to form various types ofimage viewing displays such as liquid displays, organicelectro-luminescent(EL) displays and plasma display panels (PDPs).

BACKGROUND ART

In a conventional technique, natural light emitted from a backlightenters liquid crystal cells as it is. Recently, upsizing and highdefinition of liquid crystal displays have required an increase inbacklight brightness. In addition, a number of techniques are beginningto be used for polarization of light from backlights.

In a backside of a liquid crystal cell, for example, the high-brightnesspolarizing plate with which a polarizing plate and a brightnessenhancement film are adhered together is prepared. A brightnessenhancement film shows a characteristic that reflects linearly polarizedlight with a predetermined polarization axis, or circularly polarizedlight with a predetermined direction, and that transmits other light,when natural light by back lights of a liquid crystal display or byreflection from a back-side etc., comes in. The polarizing plate, whichis obtained by laminating a brightness enhancement film to a polarizingplate, thus does not transmit light without the predeterminedpolarization state and reflects it, while obtaining transmitted lightwith the predetermined polarization state by accepting a light fromlight sources, such as a backlight. This polarizing plate makes thelight reflected by the brightness enhancement film further reversedthrough the reflective layer prepared in the backside and forces thelight re-enter into the brightness enhancement film, and increases thequantity of the transmitted light through the brightness enhancementfilm by transmitting a part or all of the light as light with thepredetermined polarization state. The polarizing plate simultaneouslysupplies polarized light that is difficult to be absorbed in apolarizer, and increases the quantity of the light usable for a liquidcrystal picture display etc., and as a result luminosity may beimproved. That is, in the case where the light enters through apolarizer from backside of a liquid crystal cell by the back light etc.without using a brightness enhancement film, most of the light, with apolarization direction different from the polarization axis of apolarizer, is absorbed by the polarizer, and does not transmit throughthe polarizer. This means that although influenced with thecharacteristics of the polarizer used, about 50 percent of light isabsorbed by the polarizer, the quantity of the light usable for a liquidcrystal picture display etc. decreases so much, and a resulting picturedisplayed becomes dark. A brightness enhancement film does not enter thelight with the polarizing direction absorbed by the polarizer into thepolarizer but reflects the light once by the brightness enhancementfilm, and further makes the light reversed through the reflective layeretc. prepared in the backside to re-enter the light into the brightnessenhancement film. By this above-mentioned repeated operation, only whenthe polarization direction of the light reflected and reversed betweenthe both becomes to have the polarization direction which may pass apolarizer, the brightness enhancement film transmits the light to supplyit to the polarizer. As a result, the light from a backlight may beefficiently used for the display of the picture of a liquid crystaldisplay to obtain a bright screen.

The suitable films are used as the above-mentioned brightnessenhancement film. Namely, multilayer thin film of a dielectricsubstance; a laminated film that has the characteristics of transmittinga linearly polarized light with a predetermined polarizing axis, and ofreflecting other light, such as the multilayer laminated film of thethin film having a different refractive-index anisotropy; an alignedfilm of cholesteric liquid-crystal polymer; a film that has thecharacteristics of reflecting a circularly polarized light with eitherleft-handed or right-handed rotation and transmitting other light, suchas a film on which the aligned cholesteric liquid crystal layer issupported etc. may be mentioned.

Therefore, in the brightness enhancement film of a type that transmits alinearly polarized light having the above-mentioned predeterminedpolarization axis, by arranging the polarization axis of the transmittedlight and entering the light into a polarizing plate as it is, theabsorption loss by the polarizing plate is controlled and the polarizedlight can be transmitted efficiently. On the other hand, in thebrightness enhancement film of a type that transmits a circularlypolarized light as a cholesteric liquid-crystal layer, the light may beentered into a polarizer as it is, but it is desirable to enter thelight into a polarizer after changing the circularly polarized light toa linearly polarized light through a retardation plate, taking controlan absorption loss into consideration. In addition, a circularlypolarized light is convertible into a linearly polarized light using aquarter wavelength plate as the retardation plate.

If the light guide plate itself has a prism structure and a prism-typecondensing sheet or the like is used together, polarized light can beemitted from a backlight, through it is slight. In such a case, thepolarization performance can be 5% or more, preferably 10% or more,further preferably 15% or more, and a direction of the emitted lightdoes not have to be in the normal direction of the backlight face. Thepolarization performance is represented by the formula: polarizationperformance=(maximum brightness−minimum brightness)/(maximumbrightness+minimum brightness). The polarization performance isdetermined by measuring, through a Glan-Thomson prism, changes inbrightness (maximum brightness and minimum brightness) of the emittedlight from the backlight in the direction of the polarization axis.

In the case that such brightness enhancement films are used, there hasbeen a problem of the amount of color shift. A various kinds of methodshave been proposed to reduce the amount of color shift (for example, seeJapanese Patent Application Laid-Open (JP-A) Nos. 11-248941, 11-248942,11-64840, and 11-64841). In these literatures, a reduction in the amountof color shift in the whole of a liquid crystal display is investigated.In JP-A Nos. 11-248941 and 11-248942, a reduction in the amount of colorshift of a brightness enhancement film is investigated. In JP-A Nos.11-64840 and 11-64841, a combination of a brightness enhancement filmand a liquid crystal panel is investigated to reduce the amount of colorshift. Conventionally, however, the amount of color shift is notsufficiently reduced in the high-brightness polarizing plate that is alaminate of a polarizing plate and a brightness enhancement film.

It is also known that a composite film of an interference multilayerlaminate and a stretched polyvinyl alcohol-based film is impregnatedwith iodine to form a high-brightness polarizing plate of a composite ofa brightness enhancement film and a polarizer (for example, see,Japanese Patent Application National Publication (Laid-Open) No.09-507308, page 12). Such the high-brightness polarizing plate canreduce the amount of color shift to some extent but can have seriousunevenness in dyeing of the polarizer, therefore it cannot be applied toimage viewing displays such as liquid crystal displays. The polyvinylalcohol-based film is stretched to 3 times or more (4 times or more or 5times or more) and controlled to have a water content of 10% or less. Inthe process of impregnating the resulting film with iodine, the speed ofiodine dyeing can vary with variations in the alignment state of thepolyvinyl alcohol-based film in the width and variations in thethickness directions and variations in the degree of crystallinity inthe width direction, so that thick parts can significantly tend to bedeeply dyed and thin parts less deeply dyed. Thus, uneven dyeing canoccur in the polarizer, and in-plane unevenness can lead to aninsufficient reduction in brightness when black viewing is displayed. Itis practically difficult to apply such a composite film to liquidcrystal displays or the like.

DISCLOSURE OF INVENTION

It is an object of the invention to provide a high-brightness polarizingplate comprising of a polarizing plate and a brightness enhancementfilm, and to suppress color shift in reduced amount.

It is another object of the invention to provide a liquid crystal panelusing such a high-brightness polarizing plate and to provide imageviewing displays such as liquid crystal displays using such ahigh-brightness polarizing plate.

The inventors have made active investigations to solve the aboveproblems and finally have found that the high-brightness polarizingplate as described below can fulfill the above objects, therebycompleting the invention. Thus, the invention is as follows:

1. A high-brightness polarizing plate, comprising: a polarizing platethat comprises a polarizer and a protective film prepared on one or bothsides of the polarizer; a brightness enhancement film; and an adhesivelayer through which the polarizing plate and the brightness enhancementfilm are laminated with the protective film interposed between them,wherein

the protective film has an in-plane retardation Re of 0 to 10 nm and athickness-direction retardation Rth of −30 to 10 nm, whereinRe=(nx−ny)d and Rth={(nx+ny)/(2−nz)}d, whereinnx is a refractive index in an X-axis direction in which a maximumin-plane refractive index is obtained, ny is a refractive index in anY-axis direction perpendicular to the X-axis, nz is a refractive indexin a Z-axis direction which is the film thickness direction, and d is anthickness (nm) of the protective film.

2. The high-brightness polarizing plate described above 1, wherein theprotective film contains (A) a thermoplastic resin having a substitutedand/or unsubstituted imide group in side chain and (B) a thermoplasticresin having a substituted and/or unsubstituted phenyl and nitrilegroups in side chain.

3. The high-brightness polarizing plate described above 1 or 2, whereinthe protective film is a biaxially stretched film.

4. The high-brightness polarizing plate described above 1 to 3, whereinthe polarizer is an iodine-containing polyvinyl alcohol-based film.

5. The high-brightness polarizing plate described above 1 to 4, whereinthe brightness enhancement film is an anisotropic reflection polarizer.

6. The high-brightness polarizing plate described above 5, wherein theanisotropic reflection polarizer is a composite of a cholesteric liquidcrystal layer and a quarter wavelength plate.

7. The high-brightness polarizing plate described above 5, wherein theanisotropic reflection polarizer is an anisotropic multilayered thinfilm capable of transmitting linearly polarized light in one directionof vibration and reflecting linearly polarized light that in anotherdirection of vibration.

8. The high-brightness polarizing plate described above 5, wherein theanisotropic reflection polarizer is a reflective grid polarizer.

9. The high-brightness polarizing plate described above 1 to 4, whereinthe brightness enhancement film is an anisotropic scattering polarizer.

10. A high-brightness polarizing plate, comprising the high-brightnesspolarizing plate described above 1 to 9 and at least one optical film.

11. A liquid crystal panel, comprising a liquid crystal cell and thehigh-brightness polarizing plate described above 1 to 10 attached to atleast one side of the liquid crystal cell.

12. A liquid crystal display, comprising the liquid crystal paneldescribed above 11.

13. An image viewing display, comprising the high-brightness polarizingplate described above 1 to 10.

Effects and Advantages

In the high-brightness polarizing plate comprising a laminate of apolarizing plate and a brightness enhancement film, transmitted lightfrom the brightness enhancement film is converted to substantiallylinearly polarized light, which enters the polarizing plate. In thiscase, the direction of the polarization axis of the brightnessenhancement film is set substantially parallel to the direction of thetransmission axis of the polarizing plate.

The polarizing plate to be used generally comprises a polarizer and aprotective film provided thereon. The protective film used has anin-plane retardation Re of approximately 0 nm such that the linearlypolarized light from the brightness enhancement film can transmit theprotective film as it is. Although protective films conventionally usedhave an in-plane retardation Re of approximately 0 nm, they haveretardation in the thickness direction. For example, a 80 μm-thicktriacetyl cellulose film has a thickness-direction retardation of −60nm, and a 40 μm-thick triacetyl cellulose film has a thickness-directionretardation of −35 nm. Protective films having in-plane birefringenceproperties are sometimes used. When such protective films are used, thedirection of the polarization axis of the brightness enhancement film isoften set parallel or perpendicular to the direction of the transmissionaxis of the polarizing plate. The inventors have found that ahigh-brightness polarizing plate comprising a laminate of the polarizingplate in which a protective film having retardation in the thicknessdirection is used and the brightness enhancement film can have a largeamount of color shift.

The development of polarization in a polarizer may be calculated fromthe imaginary part values (ke,ko) of the birefringence index of adichroic dye (such as iodine, an organic dye and a lyotropic liquidcrystal).E=EoExp(ik·z)k=(ne+no)+i(ke+ko)

Thus, if linearly polarized light enters perpendicular to the dichroicdye from any direction with the same light-intensity ratio in the frontface direction at any wavelength in the visible light range, the amountof color shift can be reduced.

In a case where the optical axis of the protective film is perpendicularto the direction of ke of the dichroic dye, for example, in a case wherethe optical axis is in the thickness direction (Z-axis direction), keaxis is in arbitrary in-plane direction (in X-Y plane) or the opticalaxis is in the in-plane direction (X-axis direction), ke axis is in thein-plane Y-axis direction or the optical axis is in the in-planedirection (X-axis direction), or ke axis is in the thickness direction(Z-axis direction), if the elevation angle is changed from the positioninclined by 45° to the in-plane X or Y direction, the apparentperpendicular relationship can be altered. Thus, it is desirable thatthe linearly polarized light derived from the brightness enhancementfilm should not be converted into omni directionalelliptically-polarized light. Therefore, it is desirable that ke axis isparallel to the optical axis of the protective film, or it is notaffected by the retardation even if ke axis is perpendicular to theoptical axis of the protective film.

Under the circumstances, the inventors have employed a protective filmwith an in-plane retardation Re of approximately 0 nm, that is 10 nm orless, and a thickness-direction retardation Rth of −30 to 10 nm, to beinterposed between the polarizing plate and the brightness enhancementfilm. Such a protective film does not affect the linearly polarizedlight from the brightness enhancement film and thus can reduce thevisual amount of color shift when white viewing is displayed on a liquidcrystal display or the like. In a preferred mode, the in-planeretardation Re is approximately 0 nm, specifically 10 nm or less, morepreferably 5 nm or less. The thickness-direction retardation Rth ispreferably from −10 to 10 nm, more preferably from −5 to 5 nm, stillmore preferably from −3 to 3 nm.

In the high-brightness polarizing plate of the invention, the polarizingplate and the brightness enhancement film are adhered together with anadhesive. In the case that a polarizing plate and a brightnessenhancement film are used, the optical properties are generally affectedby whether or not an air interface is interposed between them. Ingeneral, the brightness enhancement rate increases by 1 to 3% when noair interface is interposed, relative to when an air interface isinterposed. In such a case, however, the amount of color shift can beabout 1 to 10% higher. The optical properties also depend on thebacklight system to be used. According to the invention, theabove-defined protective film interposed between the polarizer and thebrightness enhancement film is used, and reflection at the interface isprevented by the adhesive layer, so that an improvement in brightnessduring white viewing display and a reduction in the amount of colorshift can be achieved at the same time.

When the brightness enhancement film comprises an anisotropic reflectionpolarizer of a composite of a cholesteric liquid crystal layer and aquarter wavelength plate, only about 90% of the circularly polarizedcomponent from the cholesteric liquid crystal layer can be converted bythe quarter wavelength plate. Thus, it is preferred that thethickness-direction retardation Rth of the protective film should be aslightly positive value. If the design of the quarter wavelength plateis further modified, the thickness-direction retardation Rth of theprotective film can be modified.

The high-brightness polarizing plate of the invention comprises alaminate of a brightness enhancement film and a polarizing plate thatcomprises a polarizer and a protective film provided on one or bothsides of the polarizer. In general, the polarizer of the polarizingplate according to the invention may be a product prepared by aconventional technique including the steps of subjecting a polyvinylalcohol-based film or the like to swelling, dyeing, crosslinking,stretching, water-washing, and the like as needed. In contrast to thecomposite high-brightness polarizing plate as disclosed in JapanesePatent Application National Publication (Laid-Open) No. 09-507308,therefore, the high-brightness polarizing plate according to theinvention can be prevented from causing in-plane unevenness, which wouldotherwise be caused by uneven dyeing of the polarizer, and have lowbrightness when black viewing is displayed on a liquid display or thelike.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a cross-sectional view of an example of the high-brightnesspolarizing plate according to the invention;

FIG. 2 is a cross-sectional view of another example of thehigh-brightness polarizing plate according to the invention; and

FIG. 3 is a cross-sectional view of an example of the liquid crystaldisplay according to the invention.

In the drawings, reference numerals 1 and 1′ represent polarizing plate,1 a represent polarizer, 1 b and 1 b′ represent protective film, 2represent brightness enhancement film, 2 a represent quarter wavelengthplate, 2 b represent cholesteric liquid crystal layer, (A) representadhesive layer, (B) represent backlight, (C) represent liquid crystalcell, (D) a diffusing plate, and (E) a reflective plate, respectively.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention is described below with reference to the drawings. FIG. 1is a cross-sectional view of a high-brightness polarizing plate in whicha polarizing plate (1) comprising a polarizer (1 a), a protective film(1 b) prepared on one side thereof and another protective film (1 b′)prepared on the other side thereof is laminated with a brightnessenhancement film (2) via an adhesive layer (A) with the protective film(1 b) interposed between them. The protective film (1 b) has an in-planeretardation Re of 10 nm or less and a thickness-direction retardationRth of −30 to 10 nm. FIG. 2 shows a case where a brightness enhancementfilm (2) is a composite of a cholesteric liquid crystal layer (2 b) anda quarter wavelength plate (2 a). In this composite, the quarterwavelength plate (2 a) is placed on the polarizing plate (1) side.

FIG. 3 is a cross-sectional view of a liquid crystal display comprising:a liquid crystal cell (C); a polarizing plate (1′) provided on theemission side thereof; a high-brightness polarizing plate provided onthe incidence side thereof, which comprises a polarizing plate (1) and abrightness enhancement film (2); a backlight (B); a diffusing plate (D);and a reflecting plate (E). In FIG. 3, the adhesive layer (A) isomitted. The polarizing plate (1′) placed on the emission side maycomprise a polarizer (1 a) and a protective film(s) (1 b′) provided onone or both sides of the polarizer (1 a). The protective film (1 b′) isnot limited to that having an in-plane retardation Re and athickness-direction retardation Rth in the same ranges, respectively, asthe protective film (1 b), though the protective film (1 b′) preferablyhas the same properties as the protective film (1 b).

A polarizer(1 a) is not limited especially but various kinds ofpolarizer may be used. As a polarizer, for example, a film that isuniaxially stretched after having dichromatic substances, such as iodineand dichromatic dye, absorbed to hydrophilic high molecular weightpolymer films, such as polyvinyl alcohol type film, partially formalizedpolyvinyl alcohol type film, and ethylene-vinyl acetate copolymer typepartially saponified film; poly-ene type orientation films, such asdehydrated polyvinyl alcohol and dehydrochlorinated polyvinyl chloride,etc. may be mentioned. In these, a polyvinyl alcohol type film on whichdichromatic materials such as iodine, dye is absorbed and is aligned issuitably used. Although thickness of polarizer is not especiallylimited, the thickness of about 5 to 80 μm is commonly adopted.

A polarizer that is uniaxially stretched after a polyvinyl alcohol typefilm dyed with iodine is obtained by stretching a polyvinyl alcohol filmby 3 to 7 times the original length, after dipped and dyed in aqueoussolution of iodine. If needed the film may also be dipped in aqueoussolutions, such as boric acid and potassium iodide. Furthermore, beforedyeing, the polyvinyl alcohol type film may be dipped in water andrinsed if needed. By rinsing polyvinyl alcohol type film with water,effect of preventing un-uniformity, such as unevenness of dyeing, isexpected by making polyvinyl alcohol type film swelled in addition thatalso soils and blocking inhibitors on the polyvinyl alcohol type filmsurface may be washed off. Stretching may be applied after dyed withiodine or may be applied concurrently, or conversely dyeing with iodinemay be applied after stretching. Stretching is applicable in aqueoussolutions, such as boric acid and potassium iodide, and in water bath.

During black viewing display, display unevenness or the like can becaused by uneven dyeing (variations in dyeing) of the polarizer with aniodine-containing material. For the purpose of avoiding such displayunevenness or the like, it is preferred that a polyvinyl alcohol-basedfilm or the like should be subjected to the respective steps ofswelling, dyeing (in a dyeing bath that may contain potassium iodine orthe like in addition to a dichroic dye such as iodine), crosslinking (ina crosslinking bath that may contain potassium iodine or the like inaddition to a crosslinking agent such as boric acid), stretching (in astretching bath that may contain boric acid, potassium iodide and thelike), and water-washing.

Uneven dyeing can be caused by variations in the thickness of apolyvinyl alcohol raw film (JP-A Nos. 2000-216380 and 2002-31720). Itcan be prevented, or in general applications of the polarizing plate,display unevenness can hardly be observed even if a wide range (anin-plane range of 50 cm or more, preferably of 75 cm or more, furtherpreferably of 100 cm or more) has thickness variations. During blackviewing display, unevenness can be observed if visible brightness shadepeak exist at a distance of 5 cm to 20 cm on the polarizer or thepolarizing plate, but if beyond such a distance, significant displayunevenness cannot be recognized. If it fluctuates at intervals of about5 mm or less and if uneven iodine-dyeing, which is light and shade,exists, the brightness of black viewing can only averagely increase. Theadsorption and alignment of iodine tends to depend on the thickness ofthe polyvinyl alcohol-based film, and the thicker, the more adsorptionand the higher alignment.

In the method of producing a polarizer, a polyvinyl alcohol raw filmwith less variation in thickness is preferably used. If the raw film hasa maximum thickness and a minimum thickness in a 100-400 mm in-planearea, the difference between them is 5 μm or less, preferably 3 μm orless, more preferably 1 μm or less. If the variation is greater thanthat, the film is preferably subjected to the steps of swelling inpurified water or ion-exchanged water (at 15 to 40° C. for 50 to 180seconds at a stretch ratio of 2 to 3.8), dyeing (in an aqueous solutionof iodine and potassium iodide (1:6 to 1:50) for 10 to 60 seconds with aconcentration of 0.05% to 3% (depending on the desired transmittance andthe desired polarization degree and properties) at a stretch ratio of1.2 to 2), crosslinking with boric acid (if the temperature is from 25to 45° C., the stretch ratio is from 1.1 to 2, and the concentration ofpotassium iodide is from 0 to 5%), stretching (at a boric acidconcentration of 2 to 8%, a potassium iodide concentration of 0 to 10%,a temperature of 30 to 65° C., and a stretch ratio of 1.7 to 3), andwater-washing (at a potassium iodide concentration of 2 to 10%), inwhich the total stretch ratio is preferably from 5 to 6.5. If the widthof the film is stretched to x times, the thickness and width of the filmis preferably made 1/(x)^(1/2) times, respectively. The thickness may be10% smaller than that and may be at most about 25% smaller than that.The width may be 10% wider than that and may be at most 25% wider thanthat. In a preferred mode, the stretched film is dried at 25 to 40° C.for 30 to 300 seconds such that its water content can be controlled to12 to 28% (preferably 14 to 25%).

Materials forming a protective film (1 b) are not limited, butpreferably used are composition comprising (A) a thermoplastic resinhaving a substituted and/or non-substituted imide group in a side chain,and (B) a thermoplastic resin having a substituted and/ornon-substituted phenyl group, and nitrile group in a side chain. Aprotective films comprising the thermoplastic resin (A) and (B) is hardto cause retardation, and control to make amount of in-plane retardationRe and thickness direction retardation Rth small even if the film wasstretched. The protective films comprising the thermoplastic resin (A)and (B) are described in, for example, WO 01/37007. In addition, theprotective film may also comprise other resins, when it comprisesthermoplastic resins (A) and (B) as principal components.

The thermoplastic resin (A) may have substituted and/or non-substitutedimide group in a side chain, and a principal chain may be of arbitrarythermoplastic resins. The principal chain may be, for example, of aprincipal chain consisting only of carbon atoms, or otherwise atomsother than carbon atoms may also be inserted between carbon atoms. Andit may also comprise atoms other than carbon atoms. The principal chainis preferably of hydrocarbons or of substitution products thereof. Theprincipal chain may be, for example, obtained by an additionpolymerization. Among concrete examples are polyolefins and polyvinyls.And the principal chain may also be obtained by a condensationpolymerization. It may be obtained by, for example, ester bonds, amidebonds, etc. The principal chain is preferably of polyvinyl skeletonsobtained by polymerization of substituted vinyl monomers.

As methods for introducing substituted and/or non-substituted imidegroup into the thermoplastic resin (A), well-known conventional andarbitrary methods may be employed. As examples for those methods, theremay be mentioned a method in which monomers having the above-mentionedimide group are polymerized, a method in which the above-mentioned imidegroup is introduced after a principal chain is formed by polymerizationof various monomers, and a method in which compounds having theabove-mentioned imide group is grafted to a side chain. As substituentsfor imide group, well-known conventional substituents that cansubstitute a hydrogen atom of the imide group may be used. For example,alkyl groups, etc. may be mentioned as examples.

The thermoplastic resin (A) is preferably of two or more componentcopolymers including a repeating unit induced from at least one kind ofolefin, and a repeating unit having at least one kind of substitutedand/or non-substituted maleimide structure. The above-mentionedolefin-maleimide copolymers may be synthesized from olefins andmaleimide compounds using well-known methods. The synthetic process isdescribed in, for example, Japanese Patent Laid-Open Publication No.H5-59193, Japanese Patent Laid-Open Publication No. H5-195801, JapanesePatent Laid-Open Publication No. H6-136058, and Japanese PatentLaid-Open Publication No. H9-328523 official gazettes.

As olefins, for example, there may be mentioned, isobutene,2-methyl-1-butene, 2-methyl-1-pentene, 2-methyl-1-hexene,2-methyl-1-heptene, 1-iso octene, 2-methyl-1-octene, 2-ethyl-1-pentene,2-ethyl-2-butene, 2-methyl-2-pentene, and 2-methyl-2-hexene etc. Amongthem, isobutene is preferable. These olefins may be used independentlyand two or more kinds may be used in combination.

As maleimide compounds, there may be mentioned, maleimide, N-methylmaleimide, N-ethylmaleimide, N-n-propyl maleimide, N-i-propyl maleimide,N-n-butyl maleimide, N-s-butyl maleimide, N-t-butyl maleimide,N-n-pentyl maleimide, N-n-hexyl maleimide, N-n-heptyl maleimide,N-n-octyl maleimide, N-lauryl maleimide, N-stearyl maleimide, N-cyclopropyl maleimide, N-cyclobutyl maleimide, N-cyclopentyl maleimide,N-cyclohexyl maleimide, N-cycloheptyl maleimide, and N-cyclooctylmaleimide, etc. Among them N-methyl maleimide is preferable. Thesemaleimide compounds may be used independently and two or more kinds maybe used in combination.

A content of repeating units of olefin in the olefin-maleimide copolymeris not especially limited, and it is approximately 20 to 70 mole % inall of repeating units in the thermoplastic resin (A), preferably 40 to60 mole %, and more preferably. 45 to 55 mole %. A content of repeatingunits of maleimide structure is approximately 30 to 80 mole %,preferably 40 to 60 mole %, and more preferably 45 to 55 mole %.

The thermoplastic resin (A) may comprise repeating units of theabove-mentioned olefin, and repeating units of maleimide structure, andit may be formed only of these units. And in addition to the aboveconstitution, other vinyl based monomeric repeating units may beincluded at a percentage of 50 mole % or less. As other vinyl basedmonomers, there may be mentioned, acrylic acid based monomers, such asmethyl acrylate and butyl acrylate; methacrylic acid based monomers,such as methyl methacrylate and cyclo hexyl methacrylate; vinyl estermonomers, such as vinyl acetate; vinyl ether monomers, such as methylvinyl ether; acid anhydrides, such as maleic anhydride; styrene basedmonomers, such as styrene, α-methyl styrene, and p-methoxy styrene etc.

A weight average molecular weight of the thermoplastic resin (A) is notespecially limited, and it is approximately 1×10³ to 5×10⁶. Theabove-mentioned weight average molecular weight is preferably 1×10⁴ ormore and 5×10⁵ or more. A glass transition temperature of thethermoplastic resin (A) is 80° C. or more, preferably 100° C. or more,and more preferably 130° C. or more.

And glutar imide based thermoplastic resins may be used as thethermoplastic resin (A). Glutar imide based resins are described inJapanese Patent Laid-Open Publication No. H2-153904 etc. Glutar imidebased resins have glutar imide structural units and methyl acrylate ormethyl methacrylate structural units. The above-mentioned other vinylbased monomers may be introduced also into the glutar imide basedresins.

The thermoplastic resin (B) is a thermoplastic resin having substitutedand/or non-substituted phenyl group, and nitrile group in a side chain.As a principal chain of the thermoplastic resin (B), similar principalchains as of the thermoplastic resin (A) may be illustrated.

As a method of introducing the above-mentioned phenyl group into thethermoplastic resin (B), for example, there may be mentioned a method inwhich monomers having the above-mentioned phenyl group is polymerized, amethod in which phenyl group is introduced after various monomers arepolymerized to form a principal chain, and a method in which compoundshaving phenyl group are grafted into a side chain, etc. As substituentsfor phenyl group, well-known conventional substituents that cansubstitute a hydrogen atom of the phenyl group may be used. For example,alkyl groups, etc. may be mentioned as examples. As method forintroducing nitrile groups into the thermoplastic resin (B), similarmethods for introducing phenyl groups may be adopted.

The thermoplastic resin (B) is preferably of two or more componentscopolymers comprising repeating unit (nitrile unit) induced fromunsaturated nitrile compounds, and repeating unit (styrene based unit)induced from styrene based compounds. For example, acrylonitrile styrenebased copolymers may preferably be used.

As unsaturated nitrile compounds, arbitrary compounds having cyanogroups and reactive double bonds may be mentioned. For example,acrylonitrile, α-substituted unsaturated nitriles, such asmethacrylonitrile, nitrile compounds having has α- and β-disubstitutedolefin based unsaturated bond, such as fumaronitrile may be mentioned.

As styrene based compound, arbitrary compounds having a phenyl group anda reactive double bond may be mentioned. For example, there may bementioned, non-substituted or substituted styrene based compounds, suchas styrene, vinyltoluene, methoxy styrene, and chloro styrene;α-substituted styrene based compounds, such as α-methyl styrene.

A content of a nitrile unit in the thermoplastic resin (B) is notespecially limited, and it is approximately 10 to 70% by weight on thebasis of all repeating units, preferably 20 to 60% by weight, and morepreferably 20 to 50% by weight. It is further preferably 20 to 40% byweight, and still further preferably 20 to 30% by weight. A content of astyrene based unit is approximately 30 to 80% by weight, preferably 40to 80% by weight, and more preferably 50 to 80% by weight. It isespecially 60 to 80% by weight, and further preferably 70 to 80% byweight.

The thermoplastic resin (B) may comprise repeating units of theabove-mentioned nitrites, and styrene based repeating units, and it maybe formed only of these units. And in addition to the aboveconstitution, other vinyl based monomeric repeating units may beincluded at a percentage of 50 mole % or less. As other vinyl basedmonomers, compounds, repeating units of olefins, repeating units ofmaleimide and substituted maleimides, etc. may be mentioned, which wereillustrated in the case of thermoplastic resin (A). As the thermoplasticresins (B), AS resins, ABS resins, ASA resins, etc. may be mentioned.

A weight average molecular weight of the thermoplastic resin (B) is notespecially limited, and it is approximately 1×10³ to 5×10⁶. It ispreferably 1×10⁴ or more, and 5×10⁵ or less.

A compounding ratio of the thermoplastic resin (A) and the thermoplasticresin (B) is adjusted depending on a retardation required for aprotective film. In the above-mentioned compounding ratio, in general, acontent of the thermoplastic resin (A) is preferably 50 to 95% by weightin total amount of a resin in a film, more preferably 60 to 95% byweight, and still more preferably 65 to 90% by weight. A content of thethermoplastic resin (B) is preferably 5 to 50% by weight in total amountof the resin in the film, more preferably 5 to 40% by weight, and stillmore preferably 10 to 35% by weight. The thermoplastic resin (A) and thethermoplastic resin (B) are mixed using a method in which these arekneaded in thermally molten state. The thermoplastic resin (A) and thethermoplastic resin (B) may be formed into a solution, and the solutionmay be formed into a film by a flow casting method or the like.

The material for forming the protective film may be alow-photoelastic-coefficient material such as norbornene resins.Protective films containing norbornene resins can be hard to causeretardation even if they are stressed by a dimensional change, andretardation generated by optical distortion of the protective film whenadhere to a polarizer or a brightness enhancement film can berestrained. As norbornene resins, thermoplastic saturated norborneneresins are preferred. Thermoplastic saturated norbornene resins have acyclo-olefin as main skeleton and have substantially no carbon-carbondouble bond. Examples of such thermoplastic saturated norbornene resinsinclude Zeonex® and Zeonor® manufactured by Nippon Zeon Co., Ltd. andArton manufactured by JSR Corporation.

The protective film may be a stretched film. In general, film materialscan have improved strength and mechanical toughness after they arestretched. However, many materials can generate retardation bystretching and are not useful for the protective film for the polarizer.A transparent film comprising a mixture of thermoplastic resins (A) and(B) or a norbornene resin as a main component can satisfy the aboverequirements of the in-plane retardation Re and the thickness-directionretardation Rth, even if it is stretched. Stretching treatment may beperformed with uniaxial stretching or biaxial stretching. Biaxiallystretched films are particularly preferred.

Materials for forming the protective film other than the above, which isexcellent in transparency, mechanical strength, thermal stability,water-blocking ability, isotropy, or the like, is preferably Forexample, polyester type polymers, such as polyethylene terephthalate andpolyethylenenaphthalate; cellulose type polymers, such as diacetylcellulose and triacetyl cellulose; acrylics type polymer, such as polymethylmethacrylate; styrene type polymers, such as polystyrene andacrylonitrile-styrene copolymer (AS resin); polycarbonate type polymermay be mentioned. Besides, as examples of the polymer forming aprotective film, polyolefin type polymers, such as polyethylene,polypropylene, polyolefin that has cyclo-type or norbornene structure,ethylene-propylene copolymer; vinyl chloride type polymer; amide typepolymers, such as nylon and aromatic polyamide; imide type polymers;sulfone type polymers; polyether sulfone type polymers; polyether-etherketone type polymers; poly phenylene sulfide type polymers; vinylalcohol type polymer; vinylidene chloride type polymers; vinyl butyraltype polymers; allylate type polymers; polyoxymethylene type polymers;epoxy type polymers; or blend polymers of the above-mentioned polymersmay be mentioned. The protective film is formed as cured resin layer ofheat curing type or ultraviolet curing type, such as acrylics type,urethane type, acrylics urethane type and epoxy type and silicone type.

As the opposite side of the polarizing-adhering surface above-mentionedprotective film, a film with a hard coat layer and various processingaiming for antireflection, sticking prevention and diffusion or antiglare may be used.

A hard coat processing is applied for the purpose of protecting thesurface of the polarizing plate from damage, and this hard coat film maybe formed by a method in which, for example, a curable coated film withexcellent hardness, slide property etc. is added on the surface of theprotective film using suitable ultraviolet curable type resins, such asacrylic type and silicone type resins. Antireflection processing isapplied for the purpose of antireflection of outdoor daylight on thesurface of a polarizing plate and it may be prepared by forming anantireflection film according to the conventional method etc. Besides, asticking prevention processing is applied for the purpose of adherenceprevention with adjoining layer.

In addition, an anti glare processing is applied in order to prevent adisadvantage that outdoor daylight reflects on the surface of apolarizing plate to disturb visual recognition of transmitting lightthrough the polarizing plate, and the processing may be applied, forexample, by giving a fine concavo-convex structure to a surface of theprotective film using, for example, a suitable method, such as roughsurfacing treatment method by sandblasting or embossing and a method ofcombining transparent fine particle. As a fine particle combined inorder to form a fine concavo-convex structure on the above-mentionedsurface, transparent fine particles whose average particle size is 0.5to 50 μm, for example, such as inorganic type fine particles that mayhave conductivity comprising silica, alumina, titania, zirconia, tinoxides, indium oxides, cadmium oxides, antimony oxides, etc., andorganic type fine particles comprising cross-linked of non-cross-linkedpolymers may be used. When forming fine concavo-convex structure on thesurface, the amount of fine particle used is usually about 2 to 50weight parts to the transparent resin 100 weight parts that forms thefine concavo-convex structure on the surface, and preferably 5 to 25weight parts. An anti glare layer may serve as a diffusion layer(viewing angle expanding function etc.) for diffusing transmitting lightthrough the polarizing plate and expanding a viewing angle etc.

In addition, the above-mentioned antireflection layer, stickingprevention layer, diffusion layer, anti glare layer, etc. may be builtin the protective film itself, and also they may be prepared as anoptical layer different from the protective layer.

Adhering between the polarizer and the protective film may be performedusing isocyanate-based adhesive, polyvinyl alcohol-based adhesive,gelatin-based adhesive, vinyl-latex-based adhesive, aqueous polyester,or the like. In particular, the polyvinyl-alcohol-based adhesive ispreferred. The adhesive may contain a crosslinking agent for improvingdurability. The polyvinyl alcohol-based adhesive may contain such acrosslinking agent as a metal salt, glyoxal, an alcohol-based solvent,chitosan, chitin, and melamine. The process of adhering between thepolarizer and the protective film is performed to attach them with theadhesive and drying them at a temperature of about 30 to 90° C. for 1 to5 minutes. After the process, the polarizing plate is obtained.

The brightness enhancement film may be a polarization converting elementhaving the function of dividing emitted light from a light source (abacklight) into transmitted polarized light and reflected polarizedlight or scattered polarized light. Such a brightness enhancement filmcan improve the emission efficiency of linearly polarized light by usingthe reflected polarized light or the scattered polarized light asreturning light from a backlight.

Examples of the brightness enhancement film include anisotropicreflection polarizers. For example, the anisotropic reflection polarizermay be an anisotropic multilayered thin film capable of transmittinglinearly polarized light in one direction of vibration and reflectinglinearly polarized light that in another direction of vibration. Forexample, the anisotropic multilayered thin film may be DBEF manufacturedby 3M (for example, see JP-A No. 04-268505). Examples of the anisotropicreflection polarizer also include a composite of a cholesteric liquidcrystal layer and a quarter wavelength plate. Such a composite may bePCF manufactured by Nitto Denko Corporation (for example, see JP-A No.11-231130). Examples of the anisotropic reflection polarizer alsoinclude a reflective grid polarizer. Examples of the reflective gridpolarizer include a metal reflective grid polarizer that is produced byfine metal-working and can produce reflected polarized light even in thevisible light range (for example, see U.S. Pat. No. 6,288,840) and aproduct produced by stretching a polymer containing fine metal particlesin its matrix (for example, see JP-A No. 08-184701).

Examples of the brightness enhancement film also include anisotropicscattering polarizers. For example, the anisotropic scattering polarizermay be DRP manufactured by 3M (see U.S. Pat. No. 5,825,543).

Examples of the brightness enhancement film also include polarizingelements that can achieve polarization conversion by a single pass. Forexample, such an element may use Smectic C* (for example, see JP-A No.2001-201635). The brightness enhancement film may also use ananisotropic diffraction grating.

Any adhesive may be used for adhering between the polarizing plate andthe brightness enhancement film. For example, the adhesive may beproperly selected and used from adhesives based on polymers such asacrylic polymers, silicone polymers, polyesters, polyurethanes,polyamides, polyvinyl ethers, vinyl acetate/vinyl chloride copolymers,modified polyolefins, epoxy polymers, fluoropolymers, and rubberpolymers such as natural rubbers and synthetic rubbers. In particular,adhesives having good optical transparency and good weather resistance,and heat resistance and showing suitable adhesive properties such assuitable wettability, cohesiveness and adhesion are preferably used.

The adhesive may contain a crosslinking agent depending on the basepolymer. The adhesive may also contain additives such as natural orsynthetic resins, specifically tackifier resins, fillers comprisingglass fibers, glass beads, or metal powder or any other inorganicpowder, pigments, coloring agents, and antioxidants. The adhesive mayalso contain fine particles so as to form an adhesive layer having lightdiffusing ability.

The adhesive is generally used in the form of an adhesive solutioncontaining a dissolved or dispersed base polymer or components thereofat a solids concentration of about 10 to 50% by weight. The solvent maybe properly selected and used from organic solvents such as toluene andethyl acetate and water depending on the type of the adhesive.

The above-mentioned polarizing plate may be used as ellipticallypolarizing plate or circularly polarizing plate on which the retardationplate is laminated. A description of the above-mentioned ellipticallypolarizing plate or circularly polarizing plate will be made in thefollowing paragraph. These polarizing plates change linearly polarizedlight into elliptically polarized light or circularly polarized light,elliptically polarized light or circularly polarized light into linearlypolarized light or change the polarization direction of linearlypolarization by a function of the retardation plate. As a retardationplate that changes circularly polarized light into linearly polarizedlight or linearly polarized light into circularly polarized light, whatis called a quarter wavelength plate (also called λ/4 plate) is used.Usually, half-wavelength plate (also called λ/2 plate) is used, whenchanging the polarization direction of linearly polarized light.

Elliptically polarizing plate is effectively used to give a monochromedisplay without above-mentioned coloring by compensating (preventing)coloring (blue or yellow color) produced by birefringence of a liquidcrystal layer of a super twisted nematic (STN) type liquid crystaldisplay. Furthermore, a polarizing plate in which three-dimensionalrefractive index is controlled may also preferably compensate (prevent)coloring produced when a screen of a liquid crystal display is viewedfrom an oblique direction. Circularly polarizing plate is effectivelyused, for example, when adjusting a color tone of a picture of areflection type liquid crystal display that provides a colored picture,and it also has function of antireflection. For example, a retardationplate may be used that compensates coloring and viewing angle, etc.caused by birefringence of various wavelength plates or liquid crystallayers etc. Besides, optical characteristics, such as retardation, maybe controlled using laminated layer with two or more sorts ofretardation plates having suitable retardation value according to eachpurpose.

A retardation plate may be a retardation plate that has a properretardation according to the purposes of use, such as various kinds ofwavelength plates and plates aiming at compensation of coloring bybirefringence of a liquid crystal layer and of visual angle, etc., andmay be a retardation plate in which two or more sorts of retardationplates is laminated so that optical properties, such as retardation, maybe controlled. Any of the above specified plates may be used as theretardation plate, and a homeotropic alignment liquid crystal film mayalso be used alone or in combination with any other film to form theretardation plate.

The retardation plate may be used as a viewing angle compensating filmto form a wide viewing angle polarizing plate together with thepolarizing plate laminated therewith. A viewing angle compensation filmis a film for extending viewing angle so that a picture may lookcomparatively clearly, even when it is viewed from an oblique directionnot from vertical direction to a screen.

As such a viewing angle compensation retardation plate, in addition, afilm having birefringence property that is processed by biaxiallystretching or orthogonal biaxial stretching and a biaxially stretchedfilm as inclined orientation film etc. may be used. As inclinedorientation film, for example, a film obtained using a method in which aheat shrinking film is adhered to a polymer film, and then the combinedfilm is heated and stretched or shrunk under a condition of beinginfluenced by a shrinking force, or a film that is oriented in obliquedirection may be mentioned. The viewing angle compensation film issuitably combined for the purpose of prevention of coloring caused bychange of visible angle based on retardation by liquid crystal cell etc.and of expansion of viewing angle with good visibility.

Besides, a compensation plate in which an optical anisotropy layerconsisting of an alignment layer of liquid crystal polymer, especiallyconsisting of an inclined alignment layer of discotic liquid crystalpolymer is supported with triacetyl cellulose film may preferably beused from a viewpoint of attaining a wide viewing angle with goodvisibility.

Although other optical layers laminated used in practical are especiallyno limitation, one layer or two layers or more of optical layers, whichmay be used for formation of a liquid crystal display etc., such as areflector, a transflective plate. Especially preferable polarizingplates are; a reflection type polarizing plate or a transflective typepolarizing plate in which a reflector or a transflective reflector isfurther laminated onto the elliptically polarizing plate or a circularpolarizing plate.

A reflective layer is prepared on a polarizing plate to give areflection type polarizing plate, and this type of plate is used for aliquid crystal display in which an incident light from a view side(display side) is reflected to give a display. This type of plate doesnot require built-in light sources, such as a backlight, but has anadvantage that a liquid crystal display may easily be made thinner. Areflection type polarizing plate may be formed using suitable methods,such as a method in which a reflective layer of metal etc. is, ifrequired, attached to one side of a polarizing plate through atransparent protective layer etc.

As an example of a reflection type polarizing plate, a plate may bementioned on which, if required, a reflective layer is formed using amethod of attaching a foil and vapor deposition film of reflectivemetals, such as aluminum, to one side of a matte treated protectivefilm. Moreover, a different type of plate with a fine concavo-convexstructure on the surface obtained by mixing fine particle into theabove-mentioned protective film, on which a reflective layer ofconcavo-convex structure is prepared, may be mentioned. The reflectivelayer that has the above-mentioned fine concavo-convex structurediffuses incident light by random reflection to prevent directivity andglaring appearance, and has an advantage of controlling unevenness oflight and darkness etc. Moreover, the protective film containing thefine particle has an advantage that unevenness of light and darkness maybe controlled more effectively, as a result that an incident light andits reflected light that is transmitted through the film are diffused. Areflective layer with fine concavo-convex structure on the surfaceeffected by a surface fine concavo-convex structure of a protective filmmay be formed by a method of attaching a metal to the surface of atransparent protective layer directly using, for example, suitablemethods of a vacuum evaporation method, such as a vacuum depositionmethod, an ion plating method, and a sputtering method, and a platingmethod etc.

Instead of a method in which a reflection plate is directly given to theprotective film of the above-mentioned polarizing plate, a reflectionplate may also be used as a reflective sheet constituted by preparing areflective layer on the suitable film for the transparent film. Inaddition, since a reflective layer is usually made of metal, it isdesirable that the reflective side is covered with a protective film ora polarizing plate etc. when used, from a viewpoint of preventingdeterioration in reflectance by oxidation, of maintaining an initialreflectance for a long period of time and of avoiding preparation of aprotective layer separately etc.

In addition, a transflective type polarizing plate may be obtained bypreparing the above-mentioned reflective layer as a transflective typereflective layer, such as a half-mirror etc. that reflects and transmitslight. A transflective type polarizing plate is usually prepared in thebackside of a liquid crystal cell and it may form a liquid crystaldisplay unit of a type in which a picture is displayed by an incidentlight reflected from a view side (display side) when used in acomparatively well-lighted atmosphere. And this unit displays a picture,in a comparatively dark atmosphere, using embedded type light sources,such as a back light built in backside of a transflective typepolarizing plate. That is, the transflective type polarizing plate isuseful to obtain of a liquid crystal display of the type that savesenergy of light sources, such as a back light, in a well-lightedatmosphere, and can be used with a built-in light source if needed in acomparatively dark atmosphere etc.

Moreover, the polarizing plate may consist of multi-layered film oflaminated layers of a polarizing plate and two of more of optical layersas the above-mentioned separated type polarizing plate. Therefore, apolarizing plate may be a reflection type elliptically polarizing plateor a semi-transmission type elliptically polarizing plate, etc. in whichthe above-mentioned reflection type polarizing plate or a transflectivetype polarizing plate is combined with above described retardation platerespectively.

The elliptically polarizing plate or the reflection type ellipticallypolarizing plate was laminate of the polarizing plate or the reflectiontype polarizing plate and the retardation plate with arbitrarycombination. Although the elliptically polarizing plate or the like isseparately laminated so as to form a combination of (reflection type)polarizing plate and retardation plate sequentially in manufacturingprocess of a liquid crystal display etc., an optical film, such as theelliptically polarizing plate or the like, in a form of being laminatedbeforehand has an outstanding advantage that it has excellent stabilityin quality and assembly workability, etc., and thus manufacturingprocesses ability of a liquid crystal display etc. may be raised.

The high-brightness polarizing plate of the invention may furthercomprise an adhesive layer. The adhesive layer may be used for adheringto liquid crystal cells and may also be used for lamination of opticallayers. When the high-brightness polarizing plate is adhered, theoptical axes may be set at any proper angle(s) depending on the desiredretardation properties.

Any adhesive may be used to form the adhesive layer. Examples of theadhesive may include those as specified above. The adhesive layer may beformed in the same manner.

An adhesive layer may also be prepared on one side or both sides of thepolarizing plate or the optical film as a layer in which pressuresensitive adhesives with different composition or different kind etc.are laminated together. Moreover, when adhesive layers are prepared onboth sides, adhesive layers that have different compositions, differentkinds or thickness, etc. may also be used on front side and backside ofthe polarizing plate or the optical film. Thickness of an adhesive layermay be suitably determined depending on a purpose of usage or adhesivestrength, etc., and generally is 1 to 500 μm, preferably 5 to 200 μm,and more preferably 10 to 100 μm.

A temporary separator is attached to an exposed side of an adhesivelayer to prevent contamination etc., until it is practically used.Thereby, it can be prevented that foreign matter contacts adhesive layerin usual handling. As a separator, without taking the above-mentionedthickness conditions into consideration, for example, suitableconventional sheet materials that is coated, if necessary, with releaseagents, such as silicone type, long chain alkyl type, fluorine typerelease agents, and molybdenum sulfide may be used. As a suitable sheetmaterial, plastics films, rubber sheets, papers, cloths, no wovenfabrics, nets, foamed sheets and metallic foils or laminated sheetsthereof may be used.

In addition, in the present invention, ultraviolet absorbing propertymay be given to the above-mentioned each layer, such as the polarizerfor the polarizing plate, the protective film and the optical film etc.and the adhesive layer, using a method of adding UV absorbents, such assalicylic acid ester type compounds, benzophenol type compounds,benzotriazol type compounds, cyano acrylate type compounds, and nickelcomplex salt type compounds.

A diffusion plate may also be prepared between brightness enhancementfilm and the above described reflective layer, etc. A polarized lightreflected by the brightness enhancement film goes to the above describedreflective layer etc., and the diffusion plate installed diffusespassing light uniformly and changes the light state into depolarizationat the same time. That is, the diffusion plate returns polarized lightto natural light state. Steps are repeated where light, in theunpolarized state, i.e., natural light state, reflects throughreflective layer and the like, and again goes into brightnessenhancement film through diffusion plate toward reflective layer and thelike. Diffusion plate that returns polarized light to the natural lightstate is installed between brightness enhancement film and the abovedescribed reflective layer, and the like, in this way, and thus auniform and bright screen may be provided while maintaining brightnessof display screen, and simultaneously controlling non-uniformity ofbrightness of the display screen. By preparing such diffusion plate, itis considered that number of repetition times of reflection of a firstincident light increases with sufficient degree to provide uniform andbright display screen conjointly with diffusion function of thediffusion plate.

The high-brightness polarizing plate of the present invention may bepreferably used for manufacturing various equipments, such as liquidcrystal display, etc. Assembling of a liquid crystal display may becarried out according to conventional methods. That is, a liquid crystaldisplay is generally manufactured by suitably assembling several partssuch as a liquid crystal cell, high-brightness polarizing plate and, ifnecessity, lighting system, and by incorporating driving circuit. In thepresent invention, except that the high-brightness polarizing plate bythe present invention is used, there is especially no limitation to useany conventional methods. Also any liquid crystal cell of arbitrarytype, such as TN type, and STN type, π type may be used.

Suitable liquid crystal displays, such as liquid crystal display withwhich the above-mentioned polarizing plate or the optical film has beenlocated at one side or both sides of the liquid crystal cell, and withwhich a backlight or a reflector is used for a lighting system may bemanufactured. In this case, the optical film by the present inventionmay be installed in one side or both sides of the liquid crystal cell.When installing the polarizing plates or the optical films in bothsides, they may be of the same type or of different type. Furthermore,in assembling a liquid crystal display, suitable parts, such asdiffusion plate, anti-glare layer, antireflection film, protectiveplate, prism array, lens array sheet, optical diffusion plate, andbacklight, may be installed in suitable position in one layer or two ormore layers. The backlight may use a diffusing plate, a prism sheet, alight guide plate, a cold-cathode fluorescent tube house, and the like.The arrangement order and the number regarding to the diffusing plateand the prism sheet may be not limited.

Subsequently, organic electro luminescence equipment (organic ELdisplay) will be explained. Generally, in organic EL display, atransparent electrode, an organic luminescence layer and a metalelectrode are laminated on a transparent substrate in an orderconfiguring an illuminant (organic electro luminescence illuminant).Here, an organic luminescence layer is a laminated material of variousorganic thin films, and much compositions with various combination areknown, for example, a laminated material of hole injection layercomprising triphenylamine derivatives etc., a luminescence layercomprising fluorescent organic solids, such as anthracene; a laminatedmaterial of electronic injection layer comprising such a luminescencelayer and perylene derivatives, etc.; laminated material of these holeinjection layers, luminescence layer, and electronic injection layeretc.

An organic EL display emits light based on a principle that positivehole and electron are injected into an organic luminescence layer byimpressing voltage between a transparent electrode and a metalelectrode, the energy produced by recombination of these positive holesand electrons excites fluorescent substance, and subsequently light isemitted when excited fluorescent substance returns to ground state. Amechanism called recombination which takes place in a intermediateprocess is the same as a mechanism in common diodes, and, as isexpected, there is a strong non-linear relationship between electriccurrent and luminescence strength accompanied by rectification nature toapplied voltage.

In an organic EL display, in order to take out luminescence in anorganic luminescence layer, at least one electrode must be transparent.The transparent electrode usually formed with transparent electricconductor, such as indium tin oxide (ITO), is used as an anode. On theother hand, in order to make electronic injection easier and to increaseluminescence efficiency, it is important that a substance with smallwork function is used for cathode, and metal electrodes, such as Mg—Agand Al—Li, are usually used.

In organic EL display of such a configuration, an organic luminescencelayer is formed by a very thin film about 10 nm in thickness. For thisreason, light is transmitted nearly completely through organicluminescence layer as through transparent electrode. Consequently, sincethe light that enters, when light is not emitted, as incident light froma surface of a transparent substrate and is transmitted through atransparent electrode and an organic luminescence layer and then isreflected by a metal electrode, appears in front surface side of thetransparent substrate again, a display side of the organic EL displaylooks like mirror if viewed from outside.

In an organic EL display containing an organic electro luminescenceilluminant equipped with a transparent electrode on a surface side of anorganic luminescence layer that emits light by impression of voltage,and at the same time equipped with a metal electrode on a back side oforganic luminescence layer, a retardation plate may be installed betweenthese transparent electrodes and a polarizing plate, while preparing thepolarizing plate on the surface side of the transparent electrode.

Since the retardation plate and the polarizing plate have functionpolarizing the light that has entered as incident light from outside andhas been reflected by the metal electrode, they have an effect of makingthe mirror surface of metal electrode not visible from outside by thepolarization action. If a retardation plate is configured with a quarterwavelength plate and the angle between the two polarization directionsof the polarizing plate and the retardation plate is adjusted to π/4,the mirror surface of the metal electrode may be completely covered.

This means that only linearly polarized light component of the externallight that enters as incident light into this organic EL display istransmitted with the work of polarizing plate. This linearly polarizedlight generally gives an elliptically polarized light by the retardationplate, and especially the retardation plate is a quarter wavelengthplate, and moreover when the angle between the two polarizationdirections of the polarizing plate and the retardation plate is adjustedto π/4, it gives a circularly polarized light.

This circularly polarized light is transmitted through the transparentsubstrate, the transparent electrode and the organic thin film, and isreflected by the metal electrode, and then is transmitted through theorganic thin film, the transparent electrode and the transparentsubstrate again, and is turned into a linearly polarized light againwith the retardation plate. And since this linearly polarized light liesat right angles to the polarization direction of the polarizing plate,it cannot be transmitted through the polarizing plate. As the result,mirror surface of the metal electrode may be completely covered.

EXAMPLES

The invention is more specifically described by means of Examples andComparative Examples below. It should be noted that “%” means “% byweight” in each example.

(Preparation of Polarizer)

A polyvinyl alcohol raw film (Vinylon Film VF-9P75RS manufactured byKuraray Co., Ltd.) was used which had a maximum difference of 1.2 μmbetween the maximum and minimum of variations in thickness in a 100 mmin-plane area. The raw film was subjected to a swelling process, inwhich the film was stretched at a stretch ratio of 2, while immersed inpurified water at 30° C. for 120 seconds. The film was then subjected toa dyeing process, in which the film was stretched at a stretch ratio of1.5, while immersed in a dyeing bath (an aqueous solution of iodine andpotassium iodide at a ratio of 1:10 (by weight) whose concentration wasadjusted so as to produce a final single-pass transmittance of 44.0%)for 50 seconds. The film was then subjected to a crosslinking processwith boric acid, in which the film was stretched at a stretch ratio of1.1, while immersed in a boric acid crosslinking bath (at 30° C. with aboric acid concentration of 5% and a potassium iodide concentration of2%). The film was then subjected to a stretching process, in which thefilm was stretched at a stretch ratio of 1.8, while immersed in astretching bath (at 60° C. with a boric acid concentration of 5% and apotassium iodide concentration of 5%). The film was then subjected to awater-washing process, in which the film was stretched such that thetotal stretch ratio was 6.1, while immersed in a water-washing bath(with a potassium iodide concentration of 5%) for 5 seconds. The filmwas then dried under control so as to have a water content of 20%.Relative to the raw film, the resulting stretched film (a polarizer) was42% and 39% in thickness.

(Protective Film A)

One hundred parts by weight (60% by weight) of an alternating copolymerof isobutene and N-methylmaleimide (with an N-methylmaleimide content of50% by mole and a glass transition temperature of 157° C.) and 67 partsby weight (40% by weight) of a thermoplastic styrene-acrylonitrilecopolymer with an acrylonitrile content of 27% by weight and a styrenecontent of 73% by weight were melted and kneaded into pellets. Thepellets were fed to a T-die-equipped melt extruder to obtain a 100μm-thick raw film. The raw film was stretched at a stretching speed of100 cm/minute, a stretch ratio of 1.45 and a stretching temperature of162° C. in a free-end uniaxial longitudinal stretching manner and thenstretched in a direction perpendicular to the first direction under thesame conditions in a free-end uniaxial manner to obtain a 49 μm-thickstretched film (protective film A). The protective film A had anin-plane retardation Re of 1.1 nm and a thickness-direction retardationRth of −2.8 nm. The in-plane retardation Re and thickness-directionretardation Rth of the protective film were calculated from therefractive index values nx, ny and nz at 590 nm measured with anautomatic birefringence analyzer (Automatic Birefringence Analyzer KOBRA21ADH manufactured by Oji Scientific Instruments).

The absolute value of the photoelastic coefficient of the protectivefilm A was 1.9×10⁻¹³ cm²/dye. The photoelastic coefficient is a valuedetermined from retardations under application of stress to the film.Specifically, the measurement of the photoelastic coefficient wasperformed according to the measurement method described in Memoirs ofTokyo Metropolitan Institute of Technology, Vol. 10 (1996, 12) pp.54-56.

The protective film A was immersed in an aqueous solution of 5% sodiumhydroxide at 40° C. for 2 minutes, then washed with purified water at30° C. for 1 minute, and then dried at 100° C. for 2 minutes, and theresulting saponified film was used.

(Protective Film B)

An 80 μm-thick triacetyl cellulose film (TD-80U manufactured by FujiPhoto Film Co., Ltd.) was immersed in an aqueous solution of 5% sodiumhydroxide at 40° C. for 2 minutes, then washed with purified water at30° C. for 1 minute, and then dried at 100° C. for 2 minutes, and theresulting saponified film was used. The protective film B had anin-plane retardation Re of 3 nm and a thickness-direction retardationRth of −60 nm.

(Brightness Enhancement Film A)

DBEF (an anisotropic multilayered thin film) manufactured by 3M wasused.

(Brightness Enhancement Film B)

PCF 400 (a laminate of a cholesteric liquid crystal and a quarterwavelength plate) manufactured by Nitto Denko Corporation was used.

Example 1

The protective film A was adhered to both sides of the polarizer with a5% aqueous solution of a mixture of 75 parts of polyvinyl alcohol (NH-18manufactured by The Nippon Synthetic Chemical Industry Co., Ltd.) and 25parts of glyoxal and then dried at 50° C. for 5 minutes to obtain apolarizing plate. The brightness enhancement film A and the polarizingplate were adhered together with a transparent acrylic adhesive toobtain a high-brightness polarizing plate as shown in FIG. 1. When thehigh-brightness polarizing plate was prepared, adhering was performedsuch that the absorption axis of the polarizing plate was setperpendicular to the transmission axis of the brightness enhancementfilm A.

Example 2

The protective film A was adhered to both sides of the polarizer with a5% aqueous solution of a mixture of 75 parts of polyvinyl alcohol (NH-18manufactured by The Nippon Synthetic Chemical Industry Co., Ltd.) and 25parts of glyoxal and then dried at 50° C. for 5 minutes to obtain apolarizing plate. The brightness enhancement film A and the polarizingplate were adhered together with a transparent acrylic adhesive toobtain a high-brightness polarizing plate as shown in FIG. 2. In thehigh-brightness polarizing plate, adhering was performed such that theslow axis of the quarter wavelength plate of the brightness enhancementfilm B was inclined by 45° to the absorption axis of the polarizingplate. The quarter wavelength plate side of the brightness enhancementfilm B was adhered to the polarizing plate.

Comparative Example 1

A high-brightness polarizing plate was prepared using the process ofExample 1 except that the protective film B was used in place of theprotective film A.

Comparative Example 2

A high-brightness polarizing plate was prepared using the process ofExample 2 except that the protective film B was used in place of theprotective film A.

The high-brightness polarizing plates obtained in Examples andComparative Examples were evaluated as follows. The results are shown inTable 1.

(Determination of Change in Chromaticity)

The brightness enhancement film side of each high-brightness polarizingplate was adhered to a glass plate using a laminator. The laminate wasplaced on a backlight such that the brightness enhancement film sidefaced the backlight side. The backlight was a LCD backlight for use inThinkPad A30 manufactured by IBM. Changes in chromaticity weredetermined in the normal direction (0°) of the front face of thehigh-brightness polarizing plate and in an inclined direction (70°) withrespect to the normal direction. Changes in chromaticity were measuredusing BM-7 manufactured by Topcon Corporation.

TABLE 1 0° 70° Amount of shift X Y X Y ΔX ΔY Example 1 0.3058 0.30330.3081 0.3000 −0.0023 +0.0033 Example 2 0.2905 0.3021 0.3157 0.3311−0.0252 −0.0290 Comparative 0.3102 0.3097 0.3150 0.3060 −0.0048 +0.0037Example 1 Comparative 0.2957 0.3082 0.3231 0.3396 −0.0274 +0.0314Example 2

The amount of shift was calculated from X-axis chromaticity values orY-axis chromaticity values with respect to the front face (0°) and theinclined direction (70°). Their absolute values were used forevaluation. As a result of comparing the mount of shift between Example1 and Comparative Example I and between Example 2 and ComparativeExample 2, it is apparent that the amount of shift is significantlysmaller in Examples than that in Comparative Examples.

INDUSTRIAL APPLICABILITY

The high-brightness polarizing plate of the invention comprising alaminate of a polarizing plate and a brightness enhancement film may beused alone or in combination with any other optical film to applyvarious types of image viewing displays such as liquid crystal displays,organic EL displays and PDPs.

1. A high-brightness polarizing plate, comprising: a polarizing plate;wherein the polarizing plate comprises a polarizer and a protective filmprepared on one or both sides of the polarizer, and the polarizer andthe protective film are adhered with an adhesive; a brightnessenhancement film; and an adhesive layer through which the polarizingplate and the brightness enhancement film are laminated with theprotective film interposed between them, wherein the protective film hasan in-plane retardation Re of 0 to 10 nm and a thickness-directionretardation Rth of −30 to 10 nm, whereinRe=(nx−ny)d and Rth={(nx+ny)/(2−nz)} d, wherein nx is a refractive indexin an X-axis direction in which a maximum in-plane refractive index isobtained, ny is a refractive index in a Y-axis direction perpendicularto the X-axis, nz is a refractive index in a Z-axis direction which isthe film thickness direction, and d is a thickness (nm) of theprotective film.
 2. The high-brightness polarizing plate according toclaim 1, wherein the protective film contains (A) a thermoplastic resinhaving a substituted and/or unsubstituted imide group in side chain and(B) a thermoplastic resin having a substituted and/or unsubstitutedphenyl and nitrile groups in side chain.
 3. The high-brightnesspolarizing plate according to claim 1, wherein the protective film is abiaxially stretched film.
 4. The high-brightness polarizing plateaccording to claim 1, wherein the polarizer is an iodine-containingpolyvinyl alcohol-based film.
 5. The high-brightness polarizing plateaccording to claim 1, wherein the brightness enhancement film is ananisotropic reflection polarizer.
 6. The high-brightness polarizingplate according to claim 5, wherein the anisotropic reflection polarizeris a composite of a cholesteric liquid crystal layer and a quarterwavelength plate.
 7. The high-brightness polarizing plate according toclaim 5, wherein the anisotropic reflection polarizer is an anisotropicmultilayered thin film capable of transmitting linearly polarized lightin one direction of vibration and reflecting linearly polarized lightthat in another direction of vibration.
 8. The high-brightnesspolarizing plate according to claim 5, wherein the anisotropicreflection polarizer is a reflective grid polarizer.
 9. Thehigh-brightness polarizing plate according to claim 1, wherein thebrightness enhancement film is an anisotropic scattering polarizer. 10.A high-brightness polarizing plate, comprising the high-brightnesspolarizing plate according to claim 1 and at least one optical film. 11.A liquid crystal panel, comprising a liquid crystal cell and thehigh-brightness polarizing plate according to claim 1 attached to atleast one side of the liquid crystal cell.
 12. A liquid crystal display,comprising the liquid crystal panel according to claim
 11. 13. An imageviewing display, comprising the high-brightness polarizing plateaccording to claim 1.